Global System for Mobile communications (GSM) has experienced three generations from the 1980s to the present, wherein a first generation is analog cellular system; a second generation is GSM and narrowband Code Division Multiple Access (CDMA) digital cellular system; and a third generation is wideband digital cellular system based on CDMA. Currently, the 3G Long Term Evolution (LTE) and the Fourth Generation (4G) mobile communication system both based on Orthogonal Frequency Division Multiplexing (OFDM) are being intensively studied.
However, as for the case that the work of the current 3G, the future LTE and 4G will is be operated in a frequency band higher than 2 GHz, the attenuation of radio signals in this frequency band is faster and the penetrability into buildings is worse, compared with radio signals in the traditional GSM900 frequency band. Furthermore, according to relevant statistics, 60% of the telephone traffic is generated indoors, thus, how to improve the indoor coverage of the 3G has always been a big problem for operators, and generally it is difficult for mobile terminal users to enjoy high-speed data services provided by the 3G indoors. In addition, due to the respiration effect of the 3G, the coverage of the macro node B will be reduced when traffic load of a macro cell is increased. The introduction of Femto Cell will largely share the telephone traffic of the macro cell, thus providing users in the macro cell with better voice quality. FIG. 1 shows a schematic diagram of the coverage of a macro node B and the coverage of a Femto Cell in a macro cell after the Femto Cell is introduced.
In recent years, Femto Cell is a new concept introduced by communication equipment manufacturers and communication operators according to the 3G and the evolution scheme thereof as well as the trend in mobile broadband. The concept of Femto Cell was put forward by the British company Ubiquisys among the first, and originated from insufficient coverage of the 3G in the initial stage and the challenge from the Wireless Fidelity (Wi-Fi) technique. Femto Cell provides a rapid and low-cost solution of 3G indoor coverage for households and medium or small enterprises, thus, Femto Cell is usually known as home node B with a signal coverage radius of 15-200 meters. Different from the concepts of Micro Cell and Pico Cell, only the air interface part of the Femto Cell meets the Third Generation Partnership Project (3GPP) standards or the Third Generation Partnership Project Two (3GPP2) standards; at the data backhaul side, a home fixed broadband access equipment, such as an Asymmetric Digital Subscriber Line (ADSL) modem, a cable modem etc., is applied, and the date is subsequently aggregated by a Femto Cell gateway equipment and accessed to a core network by a interface corresponding to the standard of the core network. The size of the Femto Cell is similar to an ADSL modem, which is featured with convenient is installation, automatic configuration, automatic network planning optimization, and plug-and-play. After the mobile terminal user acquires the Femto Cell from the operator, so long as the mobile terminal is powered on and is connected with a network, the Femto Cell can automatically complete Internet Protocol (IP) connection and IP distribution, and perform remote automatic software upgrading and automatic network planning, selection of the minimal interference frequency point, automatic distribution of scrambling codes, automatic establishment of a neighbor cell list and automatic regulation of transmission power.
Femto Cell is applicable to various standards, such as CDMA, GSM, Universal Mobile Telecommunications System (UMTS) etc., supports products of the Second Generation (2G), 2.5G and 3G, and has the same system and frequency band as other mobile base stations of operators, thus mobile terminals such as mobile phone can be used in the above-mentioned communication environment. Femto Cell has one carrier with the transmission power of 10-100 mW and coverage radius of 15-200 m. Femto Cell supports 4-6 active users and allows a maximal user motion speed of 10 km/h.
All the current third generation mobile communication standards are based on CDMA technology. One large advantage of the CDMA is the high spectrum reuse rate, which allows all macro node Bs to work under a unified carrier frequency, however, CDMA is also an interference-limited system; the capacity of the macro cell will be reduced when the co-channel interference of the macro cell is increased. The introduction of a great amount of unplanned Femto Cells will lead to great interference in the current macro cell. It is because that the over-high pilot power of Femto Cell will generate interference on the mobile user terminals in other macro cells, and especially for the macro cell mobile user terminals nearby the Femto Cell, the interference will become more serious. For example, a macro cell User Equipment (UE) is making a call will suffer relatively strong downlink interference when entering the coverage of the Femto Cell and it may cause dropped call when the situation becomes serious.
At present, a common solution is to configure a carrier which is different from that of the macro cell for the Femto Cell. However, when a great amount of Femto Cells are deployed, there will also be relatively strong interference between the Femto Cells; is particularly in relatively densely-populated residential areas the interference cannot be effectively solved even though a plurality of different carrier frequencies are used by the Femto Cells. Moreover, due to the lack of spectrum resources, operators can seldom have several 3G carrier frequencies; for example, the bandwidth of a single carrier frequency in the Wideband CDMA (WCDMA) standard is 5MHz and generally every operator is allocated with 2-3 frequency bands by the abroad issued WCDMA license at present, but this spectrum resources apparently cannot satisfy the need of large-scale deployment of Femto Cells.
The main function of a downlink pilot channel of the Femto Cell is to provide channel estimation and perform related demodulation for the mobile user terminal on the one hand, and provide measurement basis for the handover of the mobile user terminal between Femto Cells on the other hand. Another method to reduce interference from Femto Cells in macro cell users is to fix the value of downlink pilot power of the Femto Cell to be approximately 10% of the total transmission power of the node B.
Many researchers have learned in practice that, the setting of the fixed downlink pilot channel power is also inappropriate, because the fixed downlink pilot channel power will result in unnecessary waste during the no-load period. Thus, it remains a problem to be solved to maximally reduce the interference from the pilot power of Femto Cell in the macro cell user and the interference between the Femto Cells while still guarantees the lowest transmission power needed by the user.